1. Field of the Invention
The present invention relates to semiconductor fabrication, and more particularly to systems and methods for in-situ reflectivity degradation monitoring of optical collectors used in extreme ultraviolet (EUV) lithography processes.
2. Description of Related Art
Semiconductor fabrication typically involves dozens, or even hundreds of individual operations. In general terms, these operations can be classified as: layering, patterning, doping, and heat treatments. Among these, patterning (also referred to as “lithography”) is generally considered to be the most critical operation because it sets the physical dimensions of the resulting devices on the semiconductor wafer.
Patterning comprises a series of steps by which selected portions of material deposited on the wafer surface are removed, thus leaving a “pattern” thereon. The sequence of steps may be described as follows. First, a pattern is formed on a photomask or reticle—e.g., a glass or quartz plate having a thin layer of chrome thereon. Then, the photomask is imaged or printed onto a layer of photoresist deposited on the wafer. Etching agents remove portions of the wafer not covered by the photoresist, and the photoresist itself is removed in subsequent steps. The transfer of the pattern from the photomask onto the photoresist is performed using some form of light source or electromagnetic radiation.
The wavelength of the light source used during the lithography process is directly proportional to the size of the features that can ultimately be fabricated on the semiconductor wafer. Hence, the continuous desire to create increasingly small semiconductor devices has created a need for light sources capable of emitting very short wavelength radiation. One such light source is the extreme ultraviolet (EUV) plasma source. EUV radiation may be generated in the 13.5 nm wavelength range by a plasma-based process whereby a fuel material such as xenon, lithium, indium, tin, etc. is heated to high temperatures. This intense heat may be achieved through the use of a focused laser beam, plasma pinch electrodes applying high-energy, short-duration pulses, or the like.
Some EUV sources employ collectors to gather and redirect the radiation that they generate. For example, grazing angle collectors are typically used with gas-driven sources, whereas near normal incidence multi-layer (ML) mirror collectors, so-called distributive Bragg reflectors, are used with laser produced sources. One illustrative collector optical assembly is described in U.S. Pat. No. 6,822,251 to Arenberg et al. To increase the lifetime of a collector, the surface of the collector may be covered with a protective coating made of transition or noble metals or oxides or nitrides of such metals. Unfortunately, this coating can be gradually eroded over time due to the highly energetic ions emitted from high-power EUV sources. The loss of reflectivity invariably leads to throughput loss in lithography processes. Consequently, collectors are typically replaced once they have lost about 10% of their peak reflectivity. Furthermore, if the degradation is not uniform across the collector's surface, the collector may have to be replaced even sooner.
Conventional methods for determining a collector's reflectivity involve the removal of the collector module from the semiconductor lithography tool. The collector module is then placed within a dedicated reflectivity measurement assembly, where measurements are conducted in an expensive and cumbersome procedure. These shortcomings are not intended to be exhaustive, but rather are among many that tend to impair the effectiveness of previously known techniques for measuring the reflectivity of EUV collectors. These issues are sufficient to demonstrate that the methodologies appearing in the art have not been satisfactory, and that a significant need exists for the systems and methods described and claimed herein.